1. Field of the Invention
The present invention relates to a light-emitting device. In specific, the present invention relates to a light-emitting device in which a transistor included in a pixel uses a single-crystal semiconductor layer as a channel formation region.
2. Description of the Related Art
In recent years, technology to form a TFT over a substrate has greatly progressed, and application development for an active matrix display device has been promoted. In particular, since the field effect mobility (also called mobility) of a TFT using a polysilicon film is higher than that of a TFT using a conventional amorphous silicon film, high speed operation is possible. Therefore, development for controlling each pixel by providing a driver circuit including a TFT formed using a polysilicon film over the same substrate as the pixel is actively carried out. It is expected that, in an active matrix display device in which a driver circuit is provided over the same substrate as the pixel, various advantages can be obtained such as reduction in manufacturing cost and the size of the display device, increase in yield, and reduction in throughput.
On the other hand, TFTs formed using polysilicon have a problem that electric characteristics are likely to vary between the TFTs due to defects at crystal grain boundaries. If characteristics such as mobility or threshold values of TFTs included in pixels vary between each pixel, the amount of drain current of the TFTs varies between each pixel in accordance with the variation even when the same video signal is input. Thus, luminance of light-emitting elements also varies, resulting in display unevenness in a light-emitting device.
In order to solve such a problem, a driving method has been proposed in which the amount of current passing through an EL element can be controlled without depending on characteristics of a TFT. For example, a driving method is proposed in Reference 1 (Japanese Published Patent Application No. 2003-280587) in which constant-current driving and constant-voltage driving are switched in accordance with display.
However, as described above, there is a problem in that variation in characteristics of TFTs adversely affects display of the light-emitting device in the case where constant-current driving is performed using a TFT formed using polysilicon. In constant-voltage driving, current passing through a light-emitting element changes in accordance with a change in electric resistance of the light-emitting element which accompanies temperature change or deterioration of the light-emitting element. There is a problem in that luminance changes in accordance with a change in current since luminance of the light-emitting element is proportional to current.